1. Field of the Invention
This invention relates to colour laser marking of articles, especially security documents.
2. Description of the Related Art
Articles are laser marked in order to ensure product safety and authenticity. For example, packaging material of pharmaceuticals is laser marked to enable a consumer to verify the genuineness of a product. Laser marked security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various papers or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such articles and security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking an article, a colour change is observed by local heating of material in the bulk of the article, while in laser engraving material is removed by ablation.
Today, laser marking employed in the manufacture of security documents consists solely of a “black” laser marking method via the carbonization of a polymer, usually polycarbonate as disclosed in e.g. EP 2181858 A (AGFA). There has been considerable interest in being able to produce multicolour images through laser marking.
EP 0174054 A (POLAROID) discloses a heat sensitive element used in a thermal imaging method for forming colour images which relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible colour shift from colourless to coloured, from coloured to colourless or from one colour to another. EP 0174054 A (POLAROID) suggests using infrared absorbers that absorb radiation at 760 nm, 820 nm and 880 nm.
U.S. Pat. No. 4,720,449 (POLAROID) discloses a thermal imaging method for producing colour images on a support carrying at least one layer of a colourless compound, such as di- or triarylmethane, by conversion of electromagnetic radiation into heat. The laser beams may have different wavelengths in a range above 700 nm with at least about 60 nm apart so that each imaging layer having a different infrared absorber may be exposed separately to convert a colourless triarylmethane compound into a coloured form. U.S. Pat. No. 4,720,449 (POLAROID) suggests infrared absorbers that absorb radiation at 760 nm, 820 nm and 1100 nm, and to use cyanine, merocyanine or thiopyrylium dyes that are substantially non-absorbing in the visible region of the electromagnetic spectrum so that it will not add any substantial amount of colour to Dmin areas, i.e. the highlight areas of an image. However, no examples of infrared dyes absorbing at 1100 nm are given.
In generating multicolour images with infrared lasers, so-called colour contamination or colour fogging may occur when the infrared absorption spectra overlap too much. Less overlap results in a larger achievable colour gamut. U.S. Pat. No. 4,720,449 (POLAROID) already disclosed that the infrared dyes should have absorption maxima that are at least about 60 nm apart. For a high colour gamut, it would be desirable to have larger gaps than 60 nm possible between the absorption maxima of the infrared dyes. However, there are two problems that limit the options for doing so.
A first problem is that dyes below about 830 nm still tend to absorb light in the visual spectrum from 400 to 700 nm, resulting e.g. in a white ID card have an annoying background discoloration.
A second problem is that, although lasers above 1100 nm are available, to the best of our knowledge infrared dyes with an absorption maximum above 1100 nm having minor or no absorption in the visual spectrum are not available or known.
DE 19957001 (FEW CHEMICALS), JP 2008088426 (NIPPON KAYAKU) and U.S. Pat. No. 5,948,600 (AGFA) disclose different types of infrared dyes having minor absorption in the visual spectrum for a method for forming a heat mode image from a reducible organic metal salt. However, no dyes with an absorption maximum above 1100 nm are disclosed.
US 2006276335 (SONY) discloses a reversible multicolour thermal recording medium capable of recording and erasing repeatedly high-contrast clear images free of colour fogging without causing colour deterioration. The reversible multicolour recording medium includes recording layers containing a light-heat converting composition which generates heat upon absorption of near infrared rays having absorption peak wavelengths in the near infrared region between 750 nm and 1500 nm. The examples show reversible multicolour thermal recording media using infrared dyes having an absorption maximum between 785 nm and 980 nm only. Furthermore, the fact that the multicolour thermal recording medium is reversible makes it unsuitable for purposes of ensuring product safety and authenticity.
US 2012119171 (SHINETSU CHEMICAL CO) discloses near-infrared absorbing dyes having a specific fluorinated anion for obtaining good solvent solubility. The dyes are advantageously used in a process of fabricating semiconductor devices. Paragraph [0111] states that the dyes absorb radiation in a wavelength range of 800 to 1200 nm but the application is silent on any wavelength of an infrared absorption maximum above 1100 nm.
It is common practice to combine different types of security features in order to increase the difficulty for falsifying a security document. Because colour laser markable layers are heat sensitive, the incorporation of some security features such as e.g. a hot stamp hologram may result in an undesired colour formation in the colour laser markable layers. One possibility would be increasing the number and the thickness of the heat insulating layers used in a colour laser markable material, such as exemplified in US 2006276335 (SONY). However, a security document is by ISO-standards usually limited to a maximum thickness, e.g. 0.76 mm for ID cards as specified in ISO 7813. This leads to less desirable compromises on the background discolouration level, the number and type of included security features and the thickness of the security document.
Hence, there is firstly a need for infrared dyes having minor absorption in the visual spectrum and a maximum absorption above 1100 nm, so that the infrared absorption peaks of the different infrared sensitive recording layers can be spaced well apart in order to realize a multicolour laser markable article having a high colour gamut and minor background discoloration. Furthermore, it is also desirable that other security features can be readily combined with colour laser markable layers in a security document without requiring extra measures due to the heat sensitivity of the colour laser markable layers.